The photophysics of tryptophan in proteins will be investigated with emphasis on the origin of the complex (non-exponential or multi- exponential) fluorescence decay. Mutants of bacteriophage T4 lysozyme containing single tryptophan residues and other replacements adjacent to the tryptophan residue will be constructed. Mutations that increase the fluorescence quantum yield of tryptophan 138 have been found to decrease the complexity of the fluorescence decay. It is thus hypothesized that there is a relationship between the excited state population decay kinetics and the collisional quenching process. This may involve the existence of multiple slowly interconverting conformational states ("rotamers") or be a direct consequence of a reversibility of the quenching process. This distinction is important to the interpretation of fluorescence anisotropy experiments. Experiments are proposed that are designed to distinguish between these two mechanisms and at further enlarging our understanding of the photophysics of tryptophan - and its use as a sensitive and selective probe of protein structure and dynamics. If "rotamer" subspecies exist, they will be characterized in terms of their transition dipole orientation and interconversion rates. The presence of heterogeneity in the solid state will be investigated using Raman and fluorescence methods.